The present invention relates to a method for optical design capable of effecting precise design in light distribution for lighting equipment that uses light-emitting devices.
Featuring higher luminous flux, recently developed LEDs (light-emitting diodes) are advantageous in many applications over incandescent electric bulbs for various reasons such as long life, small power consumption and low heat generation, and hence LEDs are used in a variety of lighting device for both indoor and outdoor applications. Exemplary applications of LED lamps in automotive lighting equipment include high-mount stop lamps and rearside marker lamps that are provided for ensuring against rear-end collisions by following cars.
FIG. 10 is a vertical section showing schematically the structural layout of a high-mount stop lamp. The high-mount stop lamp indicated by a has a lens body b with a cavity. The opening in the lens body b is closed with a cover member c to form a lighting space d, which contains a substrate f with a transverse array of LEDs e (only one of which is shown in FIG. 10).
One of the inner surfaces of the lens body b which faces the LEDs e is provided with lens steps g for controlling the direction of emission of the rays of light emitted from the LEDs (only one of which is shown in FIG. 10). The lens steps are designed optically so that the distribution of light from the lamp will comply with specified standards.
When designing the distribution of light from the lighting equipment described above, the chip portion h (see FIG. 10), which is the actual source of light for LEDs e, is taken substantially as a point source of light. To design the lamp using such LEDs, ray tracing is performed on the light issuing from each LED e. The distribution of light is then generally controlled by designing the geometry of individual lens steps g.
It should be particularly mentioned that the conventional methods of optical design take the internal structure of LEDs into consideration and that only the light that issues from the LEDs is measured for analysis; as a result of which, it is difficult to perform detailed design on the distribution of light without causing waste in the quantity of light.
FIG. 11(a) shows the construction of an individual LED e. As shown, the LED e has the chip portion h in its interior, and this chip portion h is protected by a resinous lens portion i. The chip portion h is mounted in the recess k of either one of lead frames j and j' (j in the case shown in FIG. 11(a)) and the inner peripheral surface of the cavity k serves to reflect the light from the chip portion h.
The chip potion h is not negligibly smaller in size than the cavity k and the lens portion i, and hence it is difficult to insure that the effects of the geometry of the chip portion h and the reflected light from the inner peripheral surface of the cavity k in the lead frame j are dealt with precisely in a simulation of the distribution of light.
In addition, the luminous intensity distribution of each LED e is generally symmetric with respect to the optical axis, and hence the conventional methods of optical design which employ a spherical lens or a fish-eye lens step present considerable difficulties in ensuring that a luminous intensity distribution having no rotational symmetry is provided with high precision.
FIG. 11(b) is a schematic representation of luminous intensity distribution in terms of an isocandela curve drawn on a coordinate system along three axes, A--A and B--B (positional coordinate axes that are perpendicular to the optical axis of LED and which cross each other at right angles) and CD (a luminous intensity axis that crosses A--A and B--B, at right angles). As shown, the luminous intensity distribution of each LED resembles a cone, with the luminous intensity being high in the central area close to the optical axis and decreasing progressively toward the periphery away from the optical axis.
FIGS. 12 and 13 show two luminous intensity distributions for conventional lighting equipment using an LED as described above. FIG. 12 shows the relationship between the illumination angle of a light distribution pattern along the vertical line (as plotted on the vertical axis, with "UP" referring to upward and "DW" downward) and luminous intensity (as plotted on the horizontal axis CD). FIG. 13 shows the relationship between the illumination angle of a light distribution pattern along the horizontal line (as plotted on the horizontal axis, with "RH" referring to rightward and "LF" leftward) and luminous intensity (as plotted on the vertical axis CD). In the graphs, the curves indicated by solid lines show the luminous intensity distribution characteristics, whereas those indicated by one-long-and-one-short dashed lines show the characteristics specified by relevant standards.
As shown in FIGS. 12 and 13, the conventional rough method of optical design lets the luminous intensity distribution of an LED device be reflected as such in the luminous intensity distribution of the final light distribution pattern, and hence the luminous intensity is high in areas near the horizontal or vertical line and tends to decrease toward the peripheral areas. Consequently, the luminous intensity in the hatched regions exceed the specified values, causing waste in the quantity of light.
Thus, there are limits on the precision of estimation of the distribution of light from lens steps if the internal structure of a light-emitting device is not taken into account.